The present invention relates to a color cathode-ray tube used in televisions, computer displays, and the like, particularly to a color cathode-ray tube of the shadow mask type.
FIG. 15 shows a cross section of an example of a conventional color cathode-ray tube. The color cathode-ray tube 1 shown in this figure includes a substantially rectangular face panel 2 having a phosphor screen 2a formed on its inner surface, a funnel 3 connected to the back side of the face panel 2, an electron gun 4 housed in a neck portion 3a of the funnel 3, a shadow mask 6 positioned opposite to the phosphor screen 2a inside the face panel 2, and a mask frame 7 for fixing the shadow mask. Furthermore, deflection yokes 5 for deflecting and scanning with electron beams are provided on the outer peripheral surface of the funnel 3. The shadow mask 6 plays a role of color selection for three electron beams that are emitted from the electron gun 4. The letter A indicates a path of an electron beam.
In recent color cathode-ray tubes, in order to reduce reflection of external light and make a good appearance, the surface of the face panel has been made flat as shown in FIG. 15. As the face panel has a flatter surface, the shadow mask also has a flatter surface. As the surface of the shadow mask becomes flatter, the flatness of the shadow mask cannot be maintained only by supporting the body of the shadow mask with a frame.
Furthermore, when being supported only with a frame, the shadow mask is vibrated easily by a vibration from the outside, and the display image of the color cathode-ray tube is adversely affected. Therefore, as shown in FIGS. 16(a)-(b), a certain amount of tension is applied to the shadow mask (in the direction of the arrows) to stretch and fix the shadow mask in the flame.
On the other hand, during a doming phenomenon in which the surface of a shadow mask is deformed due to thermal expansion caused by electron beams crashing into the shadow mask, as the surface of the shadow mask becomes flat, displacement of an electron beam due to the doming increases, particularly in the vicinities of both ends of the image plane. Thus, in the stretching and fixing of a shadow mask as mentioned above, a practical maximum level of tension close to an elastic limit is applied to the shadow mask to absorb the thermal expansion caused by the crashing electron beams.
By such stretching and fixing, even when the temperature of the shadow mask increases, a discrepancy in the corresponding positions of the aperture for passing an electron beam in the shadow mask and of the phosphor dot on the phosphor screen can be prevented.
A shadow mask that is stretched and fixed is called a tension-type shadow mask. The tension-type shadow mask includes an aperture grill type in which many thin elements are stretched, a slot type in which many approximately rectangular apertures for passing electron beams are formed in a flat plate, and a dot type in which many circular apertures for passing electron beams are formed in a flat plate.
Furthermore, for stretching and fixing a shadow mask, there are one-dimensional and two-dimensional tension methods. The one-dimensional tension method is a method in which a tension is applied only in the longitudinal direction (up-and-down direction) of the shadow mask as shown in FIG. 16(b), and the two-dimensional method is a method in which a tension is applied in both the longitudinal and transverse directions as shown in FIG. 16(a). In the aperture grill type, the one-dimensional method is employed, and in the slot or dot type, the one-dimensional or two-dimensional method is employed.
As mentioned above, irregular color due to doming phenomenon can be prevented in the tension-type shadow mask. However, vibration of the shadow mask due to a vibration propagated from outside such as from a speaker cannot be restrained completely only by a tension applied to the shadow mask.
Therefore, in order to decrease the vibration of a shadow mask, a damper wire may be extended on the surface of the shadow mask, or may be welded onto the surface of the shadow mask. However, when using such a damper wire, its shadow is reflected in the display image of the color cathode-ray tube, so that the image quality is decreased. Various measures have been proposed up to the present to absorb vibration without causing such problems.
For example, Publication of Unexamined Japanese Patent Application (Tokuhyo) No. HEI 3-500591 has proposed a vibration attenuator comprising a rigid body fixed at a peripheral part of a shadow mask and a resistive body that is connected to the rigid body and is separate from the shadow mask. By providing such a vibration attenuator, vibration energy is extracted from the shadow mask by the rigid body integral with the shadow mask, and the extracted vibration energy is transmitted to the resistive body to be extinguished.
However, a conventional color cathode-ray tube having the above-mentioned vibration attenuator has problems as follows:
(1) In the above-mentioned vibration attenuator, the rigid body is integrated with the shadow mask by welding or the like. Thus, the rigid body itself does not serve to extinguish vibration energy, but it is merely a means for extracting vibration energy. The extracted vibration energy can be extinguished only when it is transmitted to the resistive body that is provided separately. Such a vibration attenuator has a complicated configuration, which leads to problems in cost performance and productivity.
(2) Furthermore, although the vibration attenuator is attached to a peripheral portion of the shadow mask where no aperture is formed, the shadow mask does not always vibrate at the peripheral portion depending on the frequency of the vibration propagated from outside. For example, in the case of a distribution of vibration in which the amplitude is the largest in the center portion of the shadow mask but there is almost no vibration in the right and left peripheral portions, even when a vibration attenuator is provided at a peripheral portion of the shadow mask, it cannot extract and absorb vibration energy from the shadow mask, and its effect of attenuating vibration of the shadow mask cannot be obtained sufficiently.
The present invention aims to solve the above-mentioned conventional problems, and has an object to provide a color cathode-ray tube in which vibration of an entire shadow mask can be attenuated positively with a simple structure.
In order to achieve the above object, the present invention provides a first color cathode-ray tube comprising a frame-shaped mask frame and a shadow mask in which many slot or dot apertures are formed in a flat plate, the shadow mask being stretched and fixed in the mask frame in a condition in which a tension stress is applied in one direction, wherein the amplitude in the end portions of the shadow mask is not less than a certain amount relative to the amplitude in the center portion of the shadow mask, in a vibration mode of the seventh or less order for a resonance of the shadow mask caused by a vibration propagated to the color cathode-ray tube. According to such a color cathode-ray tube, the maximum value of displacement of the shadow mask due to its vibration can be decreased.
In the first color cathode-ray tube, it is preferable that the amplitude in the end portions of the shadow mask is not less than 20% with respect to the amplitude in the center portion of the shadow mask.
Furthermore, it is preferable that the tension stress in the center portion of the shadow mask is larger than the tension stress in the end portions of the shadow mask. By having such a distribution of tension, the maximum value of displacement of the shadow mask due to its vibration can be decreased, in a resonance of a lower order mode at which the amplitude becomes large.
In a preferable color cathode-ray tube in which the tension stress in the center portion of the shadow mask is larger than the tension stress in the end portions of the shadow mask, when the tension stress in the center portion of the shadow mask is a "sgr"1 and the tension stress in the end portions of the shadow mask is a "sgr"2, it is preferable to satisfy the following relationship
"sgr"1xe2x89xa71.1"sgr"2. 
Furthermore, it is preferable that there is a maximum value of tension stress between the center portion and the end portions of the shadow mask. By having such a distribution of tension, the maximum value of displacement of the shadow mask due to its vibration can be decreased, in a resonance of a lower order mode at which the amplitude becomes large.
In a preferable color cathode-ray tube in which there is a maximum value of tension stress between the center portion and the end portions of the shadow mask, when the tension stress in the center portion of the shadow mask is "sgr"1, the tension stress in the end portions of the shadow mask is "sgr"2, and the tension stress in the intermediate portions between the center portion and the end portions is "sgr"3, it is preferable to satisfy the following relationships
"sgr"3xe2x89xa71.1"sgr"1, 
"sgr"2xe2x89xa7"sgr"1, 
and
"sgr"3xe2x89xa7"sgr"2. 
Next, the present invention provides a second color cathode-ray tube comprising a shadow mask and a mask frame for fixing the shadow mask, the shadow mask being fixed in the mask frame in a condition in which a tension is applied, which is provided with a vibration attenuator that is in contact with an end portion of the shadow mask and formed of an elastic body, and in which vibration of the shadow mask is attenuated as the shadow mask slides on the vibration attenuator. According to such a color cathode-ray tube, when the shadow mask vibrates, it slides on the vibration attenuator, so that the vibration energy is consumed by friction due to the sliding.
In the second color cathode-ray tube, it is preferable that the vibration attenuator is in contact with an end portion of the shadow mask, applying an in-plane force to the shadow mask. According to such a color cathode-ray tube, when the shadow mask vibrates, the vibration attenuator can attenuate the vibration while being in contact with the shadow mask constantly.
Furthermore, it is preferable that a dead weight for adjusting the effect of attenuating vibration of the shadow mask is attached to the vibration attenuator. According to such a color cathode-ray tube, the in-plane force that is applied to the shadow mask can be adjusted relatively easily by the dead weight.
Furthermore, it is preferable that the in-plane force is in the range of 0.3 to 3.0 gf. This range is preferable because of the following reasons: If the in-plane force is less than 0.3 gf, a frictional force necessary for the attenuation is not ensured. On the other hand, if the in-plane force is more than 3.0 gf, the frictional force becomes too strong, so that the end portions of the shadow mask may be fixed. In this case, the end portions become nodes of vibration, and the vibration is transferred to the center portion of the shadow mask, thus making the vibration even larger.
Furthermore, it is preferable that the vibration attenuator is in contact with a side surface of the shadow mask.
Furthermore, it is preferable that the vibration attenuator is inserted through a hole formed in an end portion of the shadow mask.
Furthermore, it is preferable that the shadow mask is a flat plate in which many slot or dot apertures are formed, and in which a tension is applied in one direction.
Furthermore, it is preferable that the amplitude in the end portions of the shadow mask is not less than a certain amount relative to the amplitude in the center portion of the shadow mask, in a vibration mode of the seventh or less order for a resonance of the shadow mask caused by a vibration propagated to the color cathode-ray tube. According to such a color cathode-ray tube, by positioning the vibration attenuator in an end portion, the vibration of the entire shadow mask can be attenuated effectively.
Next, the present invention provides a third color cathode-ray tube comprising a shadow mask and a mask frame for fixing the shadow mask, the shadow mask being fixed in the mask frame in a condition in which a tension is applied, which is provided with a vibration attenuator attached to the shadow mask, and in which the vibration attenuator does not have any portion adhering to the shadow mask and also is movable.
According to such a cathode-ray tube, when the shadow mask vibrates, the vibration attenuator does not vibrate integrally with the shadow mask, but vibrates separately and independently from the shadow mask, while repeating contacting and sliding with the shadow mask or temporarily being separated therefrom. Thus, vibration energy of the shadow mask is consumed by the friction caused by such contacting and sliding between the shadow mask and the vibration attenuator, so that the vibration of the shadow mask can be attenuated.
In the third color cathode-ray tube, it is preferable that the vibration attenuator is inserted through a hole formed in the shadow mask. According to such a color cathode-ray tube, the vibration attenuator can be attached to the shadow mask in such a way it is movable with a simple structure.
Furthermore, it is preferable that the vibration attenuator is a ring-shaped member.
Furthermore, it is preferable that the vibration attenuator is a frame-shaped member.
Furthermore, it is preferable that the mass of the vibration attenuator is in the range of 0.02 to 5.0 g. This range is preferable because of the following reasons: If the mass is less than 0.02 g, a frictional force necessary for the attenuation is not ensured. On the other hand, if the mass is more than 5.0 g, vibration at the attached portion may be restrained from the beginning, and in this case the vibration is transferred to other portions.
Furthermore, it is preferable that the vibration attenuator is attached to a portion of the shadow mask where no apertures for passing electron beams are formed.
Furthermore, it is preferable that the vibration attenuator is attached to a portion of the shadow mask where apertures for passing electron beams are formed.
Furthermore, it is preferable that a second vibration attenuator other than the above-mentioned vibration attenuator (first vibration attenuator) is provided for attenuating the vibration of the first vibration attenuator by contacting with it when it is vibrating. According to such a color cathode-ray tube, the effect of attenuating vibration can be more enhanced.
Furthermore, it is preferable that the shadow mask is a flat plate in which many slot or dot apertures are formed and a tension is applied in one or two directions.
Furthermore, it is preferable that the amplitude in the end portions of the shadow mask is not less than a certain amount relative to the amplitude in the center portion of the shadow mask, in a vibration mode of the seventh or less order for a resonance of the shadow mask caused by a vibration propagated to the color cathode-ray tube. According to such a color cathode-ray tube, by positioning the vibration attenuator in an end portion, vibration of the entire shadow mask can be attenuated effectively.